1. Field of the Invention
The present invention is generally directed to a substrate or method for a more efficient removal of sulfur compounds from the outlet of a fuel processor. In particular, this invention provides an improved sorption capacity which enables the sulfur clean-up unit to operate for a longer duration. Additionally, this invention provides for a regenerable, energy efficient, and lightweight sorption system for the removal of sulfur in fuel cell applications.
2. Description of the Related Art
Absorption, in chemistry, is a physical or chemical phenomenon; a process in which atoms, molecules, or ions enter some bulk phase—gas, liquid or solid material. Absorption is the taking of molecules of one substance directly into another substance. Absorption may be either a physical or a chemical process; physical absorption involving such factors as solubility and vapor-pressure relationships, and chemical absorption involving chemical reactions between the absorbed substance and the absorbing medium. In sum, the process of absorption comprises the molecules of one material being taken up by the volume, not by surface, of another material.
Adsorption is a different process from absorption since, with adsorption, the molecules of a first substance adhere only to the surface of the second substance. Adsorption is a process that occurs when a gas or liquid solute accumulates on the surface of a solid or a liquid (adsorbent), forming a film of molecules or atoms (the adsorbate). It is different from absorption in which a substance diffuses into a liquid or solid to form a solution. Adsorption is the gathering of matter only. The matter collects on the surface of the adsorbing material; it does not enter the interior.
As is known to those skilled in the art, and as set forth above, adsorption is adhesion of the molecules of liquids, gases, and dissolved substances to the surfaces of solids, as opposed to absorption, in which the molecules actually enter the absorbing medium. A more general term used in the art is “sorption” which covers adsorption, absorption, and ion exchange.
As is also known in the prior art, Zeolites are natural or synthetic crystalline aluminosilicates which have a repeating pore network and release water at high temperature. They are manufactured by hydrothermal synthesis of sodium aluminosilicate or another silica source in an autoclave followed by ion exchange with certain cations (Na+, Li+, Ca2+, K+, NH4+). The channel diameter of zeolite cages usually ranges from 2 to 9 Å (200 to 900 pm). The ion exchange process is followed by drying of the crystals, which can be pelletized with a binder to form macroporous pellets. Zeolites are applied in the drying of process air, CO2 removal from natural gas, CO removal from reforming gas, air separation, catalytic cracking, and catalytic synthesis and reforming.
Conventional sulfur sorbents are usually based on pellets or extrudates. For example, zinc oxide (ZnO)-based pellet or extrudate is commonly used as H2S sorbent. The ZnO pellet performance strongly depends on pore and lattice diffusion, and is low at low temperature (<350° C.) due to the high activation energy of the lattice diffusion process. One way to improve the low temperature activity is to adjust the pore structure and the surface area of the pellet. On the other hand, sorbent regeneration requires the pellet/extrude be able to survive frequent sorption/desorption cycles thus requires high crush strength and low dusting. High pore volume and high crush strength are conflicting requirements. Therefore, sorbent designs often have to balance these issues.
In one study performed by B. K. Chang, Y. Lu, H. Yang, and B. J. Tatarchuk, as reported in the Journal of Materials Engineering and Performance, volume 15 (2006) (hereinafter referred to as “Chang”), the authors describe the development a system consisting of microsized zinc oxide (ZnO)/SiO2 sorbent particulates supported on a microfibrous, glass fiber material and its application for the H2S removal from the outlet/reformate stream of a fuel processor. The system in their study showed a significant improvement in the ZnO utilization compared to the typical ZnO extrudates bed due to the use of small-sized support particulates, which promotes the high contacting efficiency and high accessibility of the ZnO. Furthermore, their system provided significant improvement in the regenerability aspect compared to the ZnO extrudates bed.
One problem associated with the ZnO sorbent loading in the system described above is the limited loading capacity of only ˜1050 mg of ZnO per in3 of bed (calculated based on the information given in Table 1 of Chang). In addition to a high sulfur sorption capacity (weight basis) and high ZnO utilization, both of which were shown in Chang, a high ZnO sorbent loading in an H2S removal unit is important in order to achieve a high overall volumetric sorption capacity (mg of H2S uptake per in3 of H2S removal bed). Additionally, the Chang system implemented an external heating element during the regeneration process which leads to a less energy efficient, heavier sulfur sorption system.
In a study performed by H. Yang, Y. Lu, and B. J. Tatarchuk, as reported in the Journal of Power Sources, volume 174 (2007) (hereinafter referred to as “Yang”), the authors further describe the development of glass fiber entrapped ZnO/SiO2 sorbent (GFES) to remove sulfur (H2S) from the reformate/outlet stream of a fuel processor with the benefits of a longer breakthrough time, higher ZnO utilization, and improved regenerability compared with the commercial ZnO extrudates. However, as with Chang, this system provides a limited loading of ZnO per in3 of bed; while a high ZnO sorbent loading in an H2S removal unit is important in order to achieve a high overall volumetric sorption capacity.
In yet another study, G. Alptekin, S. DeVoss, M. Dubovik, J. Monroe, R. Amalfitano, and G. Israelson, Journal of Materials Engineering and Performance, volume 15 (2006) (hereinafter referred to as “Alptekin”), the authors describe the development of a sorbent material for natural gas desulfurization. Their test results showed that the sorbent can remove sulfur compounds at ambient temperature with relatively high capacity. The sorbent can also be regenerated by the temperature swing regeneration procedure.
Although the Alptekin sorbent material showed an improvement over other sorbents when operating at ambient temperature (See Alptekin, Table 2), the sulfur sorption capacity of 3.1 wt. % is still very low compared to the commercial ZnO sorbent material (i.e., the sulfur sorption capacity of a ZnO extrudates bed is ˜10-15 wt. % when operating at its optimum temperature of 300-400° C.). The authors did not mention the sulfur sorption capacity of the Alptekin sorbent material when operating at 300-400° C. The low sulfur sorption capacity of the sorbent bed will result in a shorter bed lifetime, which is not desirable for practical applications. Furthermore, as with Chang, the system requires an external heater to provide heat during the regeneration process, which results in a longer regeneration time.
In other prior art studies, I. I. Novochinskii, C. Song, X. Ma, X. Liu, L. Shore, J. Lampert, and R. J. Farrauto, as reported in Energy and Fuels, volume 18 (2004) (hereinafter, “Novochinskii”), the authors describe the development and testing of a unique, modified ZnO sample with a different morphology than a commercially available ZnO sample in the year of 2003. The papers disclosed the study on ZnO particles and extrudates, and the application and coating of the modified ZnO sample on a monolith. The ZnO bed was used to remove H2S from the outlet stream of a fuel processor for fuel cell applications. This study also found that the sorbent-coated monolith gave a higher sulfur sorption capacity compared to the extrudates bed of the same sorbent material under the same operating conditions.
One problem associated with Novochinskii is that in the extrudates form, the modified ZnO sorbent had a maximum sulfur sorption capacity of only 2.81 wt. % (i.e., 0.0281 g of S sorbed per gram of ZnO), which is lower than the optimum capacity of current (2007) commercial ZnO pellet bed at similar operating conditions. Upon coating the modified ZnO sorbent on a monolith substrate, the sulfur sorption capacity could be increased to 3.55 wt. % (i.e., 0.0355 g of S sorbed per gram of ZnO). Additionally, the ZnO sorbent loading in the monolith bed was only ˜1070 mg of ZnO per in3 of bed (calculated based on the information given in Novochinskii, Table 3), which is much lower than that which may be achieved with the present invention.
In yet another study, R. B. Slimane and B. E. Williams, as reported in the Industrial and Engineering Chemistry Research, volume 41, (2002) (hereinafter, “Slimane”), the authors describe a sorbent synthesis technique developed at GTI for the preparation of ZnO-based regenerable sulfur sorbents. The capability to produce sintering at low temperatures resulted in materials with high surface areas, small pore sizes, and very high attrition resistance. However, the sulfur sorption capacity (weight basis) in the powder form is either lower or similar with the capacity obtained using the current (2007), commercially available ZnO pellets. Moreover, this prior art does not describe the bed utilization when testing the sorbent materials in extrudates or pellets form. Sulfur sorption process using the powder form is not practical due to an extremely high pressure drop across the bed. Finally, as with many of the other prior art systems, the Slimane sulfur removal bed requires an external heater to provide heat during the regeneration process, which results in a longer regeneration time.
As described in U.S. Pat. No. 7,141,092 to Roychoudhury, et al. (“the '092 patent”) there are several methods known in the prior art for raising the temperature of the process to a required temperature range including heating the flowstream or the sorbent structure by employing an external heat source. However, non-uniform heat distribution within a fixed-bed substrate or other sorbent structure negatively impacts the efficiency of the process. In addition, the time it takes for an external heat source to raise the temperature of the sorbent structure, and thereby raise the temperature of the sorbent and the working fluid, further negatively impacts the efficiency of the process.
As further taught in the '092 patent, CO2 and trace contaminants can be removed within a single-unit employing a sorption bed comprising ultra-short-channel-length metal meshes coated with zeolite sorbents. The adsorption process of the '092 patent was designed for spacecraft cabin air quality control and the removal of environmental contaminants in space flight applications. The '092 patent teaches the use of zeolites for the adsorption of CO2 and trace contaminants.
Accordingly, based on the shortcomings of the prior art, it is an object of the present invention to provide a sulfur removal unit in a fuel processor that has a higher volumetric sorption capacity than the current pellets or extrudates bed system to allow for a longer lifetime at the same unit size. It is also an object of the present invention to provide a regenerable, efficient sulfur removal unit that can be regenerated via a direct, resistive heating method which will enable a faster and logistically desirable periodic regeneration and will lead to a more energy efficient sulfur removal unit.